1. Field of the Invention
The present invention relates to a crystalline polymer microporous membrane having a high filtration efficiency, used for microfiltration of gases, liquids, etc.; a method for producing the crystalline polymer microporous membrane; and a filter for filtration.
2. Description of the Related Art
Microporous membranes have long since been known and are widely used for filters for filtration, and the like (refer, for example, to “Synthetic Polymer Membrane” authored by R. Kesting, published by The McGraw-Hill Companies, Inc.). Examples of such microporous membranes include those produced using cellulose esters as raw materials (refer, for example, to U.S. Pat. No. 1,421,341), those produced using aliphatic polyamides as raw materials (refer, for example, to U.S. Pat. No. 2,783,894), those produced using polyfluorocarbons as raw materials (refer, for example, to U.S. Pat. No. 4,196,070), and those produced using polypropylene as a raw material (refer, for example, to West Germany Patent No. 3,003,400).
These microporous membranes are used for filtration and sterilization of cleaning water for use in the electronics industry, water for medical uses, water for pharmaceutical production processes and water for foods and drinks. In recent years, the microporous membranes have been coming to be used for wider purposes and in larger amounts, and note has been taken of microporous membranes which are highly reliable in terms of trapping particles. Among these, microporous membranes made of crystalline polymers are superior in chemical resistance, and microporous membranes produced using polytetrafluoroethylene (PTFE) as a raw material, in particular, are superior in both heat resistance and chemical resistance and thus are attracting more and more demands.
Meanwhile, Japanese Patent Application Laid-Open (JP-A) No. 11-515036 proposes a method for producing a porous polytetrafluoroethylene product, including a step of uniaxially stretching a tape at a temperature below the crystalline melting point of a polytetrafluoroethylene component and increasing the temperature of the tape to a temperature above the crystalline melting point of the polytetrafluoroethylene component so as to fix the stretched tape in an amorphous form, and a step of stretching the tape in a direction perpendicular to the direction of the above-mentioned stretching at a temperature above the crystalline melting point of the polytetrafluoroethylene component. According to this proposal, it is possible to increase the flow rate in filtration. However, this proposal presents such a problem that the filtration capability per unit area of a microporous membrane is low (in other words, the filtration life is short).
Meanwhile, JP-A No. 2007-332342 proposes a method for producing a crystalline polymer microporous membrane, including a semi-baking step in which thermal energy is provided to a surface of an unbaked film such that the film has a temperature gradient with respect to its thickness direction. According to this proposal, multistage filtration is enabled by pores with an asymmetrical structure, thereby making it possible to lengthen the filtration life of the microporous membrane. However, this proposal presents such a problem that a microporous membrane with a high flow rate cannot be produced. Moreover, the method for producing a crystalline polymer microporous membrane by means of the semi-baking step does not enable the membrane to have a temperature gradient unless the membrane is thick to some extent, and it is difficult to reduce its thickness in reality.